The present invention concerns a planar direct drive (also known as a planar motor), which includes a position measuring system for controlling the travel of the rotor. Direct drives of this type have a passive unit with a planar running surface, in which magnetic flux regions are integrated. This passive unit constitutes the stator of the motor. In addition, there is at least one active unit (rotor) with coil systems for generating a variable magnetic flux. The active unit can move on the running surface of the passive unit. Furthermore, the direct drive has a bearing unit, which allows frictionless two-dimensional relative motion of the active and passive units.
DE 195 13 325 A1 describes, for example, a Hall sensor position measuring device for use in linear and planar motors. To this end, several Hall sensors are integrated in the active unit, which detects variations in magnetic field strength arising from the relative motion of the active and passive units. The tooth pitch on the running surface of the passive unit constitutes the measurement standard, which is scanned by the sensors. However, this also limits the positioning accuracy of the planar motor, which depends directly on the tooth pitch and its accuracy. Positioning accuracies in the range of 20-40 μm can be realized by the use of these well-known position measuring methods. Another problem is that the positioning accuracy also depends on environmental conditions, especially the temperature of the passive unit. The running surface of the passive unit consists to a great extent of metallic materials (especially soft iron material), which have a relatively high temperature coefficient. Due to ambient temperature variations and also due to self-heating produced by the operation of the motor, the running surface can experience considerable expansion, so that the measurement standard itself experiences linear expansion that is significant with respect to the desired positioning accuracy. If direct drives of this type are to be used for positioning tasks with high accuracy requirements, errors of this type can no longer be tolerated. Ongoing miniaturization in many technical fields requires rather an increase in positioning accuracy, which cannot be achieved with previously known integrated position measuring systems.
Higher measuring accuracies can be realized, for example, if external measuring systems are used, for example, with the use of laser interferometers or glass scales. In this case, however, either large and heavy position measuring standards must be carried by the active unit, or coupling to external measurement standards is necessary. This type of arrangement is problematic, especially with planar motors, since the active units are meant to be moved two-dimensionally as desired on a larger running surface. The use of optical measuring methods is also possible only to a limited extent, since the path of working beams would frequently be disturbed by other elements (for example, other active units moving on the running surface, cables, and the like).
In this connection, DE 202 10 305 U1 describes a positioning table of a high-speed linear motor, which uses a position measuring system that consists of a reading head mounted on the primary part and a strain gage mounted on the rotor. A problem here is that the bearing gap must be enlarged to prevent damage to the strain gage during movement. However, this drastically reduces the maximum motive force. Furthermore, the reading head is mounted in a stationary way on a certain place of the primary part, so that an exact position determination is possible only in the area of the adhesively mounted strain gage. The position measuring system is not available beyond the travel range detected by the strain gage.